Salt concentration Raman band intensity

Doubts have been cast on these conclusions by Janssen (1961) and Cook (1964), who studied infrared spectra of the salts of dimethyl-acetamide. The strong C=0 band in the solid amide appears at 1639 cm-1. In the solid hydrochloride a band appears at 1693 cm-1, which was assigned to the C=N bond in the O-protonated cation (Janssen, 1961), but could equally be due to the C=0 vibration in the N-protonated cation. Raman spectra of dimethylacetamide (2 m ) in cone, hydrochloric acid show the same intense band at 1693 cm-1 (de Loze et al., 1972). In view of the high concentrations of both the amide and the acid, it is probable that the amide is present in these solutions largely as the O-protonated cation, so that the assignment of this band to the C=N frequency seems likely, especially for a tertiary amide. Infrared and Raman spectra are thus capable of interpretation in terms of both N- and O-protonation, depending to some extent on the amide (primary, secondary or tertiary),. and cannot provide unambiguous evidence for the site of protonation. 

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